The present work seeks to advance quantitative understanding of semicircular canal biomechanics under both physiological and pathological conditions with the goals of 1) improving the assessment and treatment of benign paroxysmal positional vertigo (BPPV) and 2) furthering the basic understanding of micromechanical and hair cell electro-mechanical factors in the process of angular motion sensation. Specifically, the work aims to: quantify the three-dimensional substrates of canalolithiasis and its relationship to BPPV, determine micromechanical contributions to spatial diversity in hair-cell activation and afferent response dynamics and, establish the role of hair-cell electro-mechanical feedback in the crista ampullaris. The first aim will detail biomechanical and neural responses under conditions of experimentally induced canalolithiasis during provocative diagnostic tests and during canalith repositioning procedures. The second aim will determine micromechanical and hair cell transduction current contributions to the diverse neural code transmitted from the semicircular canals to the brain. Regional differences in cupula motion will be measured during sinusoidal and step stimuli using a laser interferometer. The spatial distribution of haircell transduction currents will be mapped across the sensory epithelium and compared to micromechanical data to determine the relative contributions of each. Experiments to investigate the role of hair cell/bundle electro-motility will be included. The third aim will investigate the influence of activation of the efferent vestibular system and, separately, electrical polarization of the endolymph on hair-cell and micromechanical responses. Detailed mathematical models will be used extensively to interpret results and direct the experimental studies. Computer models and animations showing responses to clinical diagnostic tests and canalith repositioning procedures will be generated. Results are expected to have immediate relevance to health and the human condition through the assessment and treatment of classical and non-classical BPPV as well as long-term significance enhancing basic understanding of semicircular canal micro-mechanics, haircell transduction and electro-mechanical feedback.